128 The Life Story of the Fish 



cases, a rapid one, and so the fish must have time to make 

 the adjustment gradually. 



At this point in their reasoning, a great light dawned on 

 certain scientists, and they brought forth the theory that it 

 was the duty of the air-bladder to keep the fish within certain 

 depth limits, or to prevent it from making depth changes too 

 suddenly. This is at first sight a tempting idea. Rapid changes 

 of altitude bring about undesirable disturbances in our in- 

 ternal workings. And yet for us to go up to ten thousand feet 

 from sea-level is no more, so far as change in pressure is 

 concerned, than for a fish to come up from a ten-foot bottom 

 to the surface, an act which he can accomplish in a few sec- 

 onds. The air-bladder is there to prevent him from over- 

 indulging this ability and thus unknowingly injuring himself. 

 If he comes up too far or too fast, the air-bladder expands 

 and pokes him in the stomach. If he goes down too far or 

 too fast, his stomach pokes him in the air-bladder. In either 

 case, he has an ache in his insides which warns him that 

 something is wrong, and that he had better go back where 

 he came from. 



There is no question that something of this kind does 

 happen. An Italian experimenter found that if he increased 

 the pressure in the air-bladder by injecting sterile, air-free 

 water, the fish tried to swim up, and if he reduced it the fish 

 tried to swim down. In other words, increased pressure gave 

 the fish the sensation that he was too far down and had better 

 come up, and vice versa. This proves that an air-bladder out 

 of adjustment with the depth does warn the fish to seek a 

 different level, but it does not prove that the same fish, if it 

 had no air-bladder, would have any disturbance in its other 

 organs at that depth. The whole theory of the air-bladder 

 serving as a warning pressure gauge is wrecked by the number 

 of fish which have no air-bladder at all, and which safely 

 make great changes in their level just the same. The halibut 



